To overcome the problem of active device switching losses in power converters, while enabling operation at higher switching frequencies, soft-switching converters have been developed. In general, there are two types of soft-switching, or resonant, converters: zero-voltage switching and zero-current switching. Zero-voltage switching involves switching the active devices when there is zero voltage thereacross. Zero-current switching involves switching the active devices when there is zero current therethrough. Unfortunately, however, higher voltage or current stresses generally result from operation of such soft-switching power converters, necessitating the use of devices with higher voltage or current ratings, respectively.
Recently, an LC resonant snubber circuit triggered by auxiliary switching devices was proposed to minimize the high dynamic stresses encountered when turning on and off the main switching devices in an inverter. As described by W. J. McMurray in a paper entitled "Resonant Snubbers with Auxiliary Switches", 1989 IEEE-IAS Conference Proceedings, pp. 829-834, each of the main switching devices of an inverter having two devices per phase leg, or pole, is coupled in parallel with a sufficiently large capacitor to achieve substantially zero-voltage turn-off conditions. The control initially provides a turn-off signal to one of the main switching devices of an inverter pole, and subsequently triggers an auxiliary switching device to provide a temporary path to take over the high-stress, turn-on duty from the other main switching device of the inverter pole in a manner that leaves no energy trapped after switching. In particular, the LC resonant circuit ideally swings the output voltage from one power rail to the other, at which time the opposite main switching device is turned on. Advantageously, this resonant snubber circuit topology does not impose any voltage or current penalties on the main devices. Moreover, each inverter phase can be controlled independently using pulse width modulation, resulting in converter waveforms having low harmonic distortion.
In practice, however, using the hereinabove described control method, the resonant output voltage may fall short of the opposite rail voltage due to component resistances, device conduction losses and inadequate forcing potential. As a result, the next switching device in the inverter pole to be turned on may be switched at the peak of the resonating voltage, and hence must absorb some switching losses due to the non-zero voltage turn-on, including the energy dump from the parallel capacitor. Hence, although the resonant snubber circuit topology exhibits reduced main device stresses during switching instants and substantially reduced switching losses, it is desirable to reduce switching losses and associated EMI noise even further and thus more closely approach truly "lossless" switching.